Ultrasonic ranging systems use airborne high frequency sound waves to detect target objects. In these systems, high frequency sound waves are generated and transmitted to an object. The transmitted sound waves then bounce off the object and return to their source as an echo. The distance from the source of the sound waves to the object can then be measured based on the speed of sound and the time it takes for the sound wave to travel to an object and return to the source.
Although ultrasonic sound with frequencies ranging from 20 to 200 kHz can be used for a wide variety of ranging and sensing applications, when designing an ultrasonic-based system, a great care must be taken to compensate for the many variables involved. An example of such variables includes dynamic temperature changes in the atmosphere. If atmospheric temperature is not properly compensated for, an erratic system operation will result. An uncompensated system could, for example, exhibit undesirable traits such as a range that varies, blind spots, moving blind spots, a target that is detected one time and not the next, unwanted target acquisitions, and false target acquisitions.
In addition, a reliable ultrasonic system needs to account for acoustic properties that are affected by the environmental dynamics. Such acoustic properties include variation in the speed and wavelength of sound in air over temperature; variation in sound attenuation based on frequency, temperature, and humidity and over distance; variation in the return echo, target strength, based on target distance, shape and composition; turbulence in the detection zone; effects of background noise; and sound radiating pattern, beam angle, of the selected ultrasonic transducer.
For example, speed of sound varies from 1041 feet/second (“ft/sec”) at −10 degrees Fahrenheit (“F”) to 1172 ft/sec at 110 F. This presents a change of 12.5% and would result in an apparent change in measurement of 2.5 ft over a 20 ft distance. Further, wavelength is defined as the speed of sound in inches/second (“in/sec”) divided by frequency, w1=c/f. Since the speed of sound, c, changes over temperature so does the wavelength. Solving for wavelength, it can be seen that a 40 kHz signal at −10 F has a wavelength of 0.3123 inch, while at +110 F, the wavelength is 0.3516 inch. For reflection to occur, the wavelength should be small compared to the dimension of the target because the larger the target in comparison to wavelength, the stronger the return. Thus it can be seen that as temperature goes up, wavelength goes up and the amount of reflection goes down. That is, the target strength diminishes.
Sound propagates through air by causing air molecules to collide with each other pushing the sound along like a wall of dominoes. As sound travels through air, these collisions result in friction loss, higher frequency means more collisions, hence greater loss. Complicating the issue is the fact that density and composition of the medium varies with temperature and humidity. Further, the medium behaves differently above 50 kHz. A useful approximation for figuring maximum attenuation up to 50 kHz and above 50 kHz may be used. While useful for approximating maximum ranges, however, the actual temperature humidity attenuation is highly non-linear. Accordingly, this attenuation also needs to be compensated for or target strength will vary radically over temperature causing erratic target acquisition.
As described above, because both sound and target objects have complex properties, many considerations need to be taken into account when building an ultrasonic system. Accordingly, there is a need for a properly designed ultrasonic system that compensates for the environmental factors such as dynamic temperature changes.